I'll preface this with: I have little doubt that they're charging $5 because they can, and it goes no further than that.
However! They might be using this as an insurance policy of sorts, follow with me:
- Your number is no different than a celebrities to look at/in their database/etc.
- You pay the same for non-listing as does said celebrity
- Unlike you, the celebrity could possibly win a lawsuit for a decent chunk of money if the company (through negligence) publishes their number.
So given that accidents happen, humans are prone to error, etc. etc. It is just possible that you're paying $0.50 a month for the actual service, and $4.50 a month for their eventual legal costs due to a fuckup.
As I said above though, when phone companies charge thousands of times reasonable rates for SMS and such I hardly expect that it goes any further than "because they can".